Anti-theft light on?

Turn the key to start and leave the key on. Tell me if the theft light flashes continuously.

The aftermarket alarm you had installed is probably not the problem. The problem I find usually is from the factory alarm. This is how to get it started. Make sure battery is charged. Crank engine then leave the key in the on position and watch the theft light. In about ten minutes the light will stop flashing then go out. When it goes out, turn the key off then attempt to start the car, it should start. Your problem is usually the ignition switch or the ignition cylinder. This procedure is a temporary fix, it might last for a month or a week. Let me know how you make out.

Try this.

General instructions on pass-lock relearn:

So the engine cranks but it wont start? IS the security light flashing? If so try this
GM PASS LOCK RELEARN- Ignition to "run" position, try to start and let key return to "run" position
Approx ten minutes security light will go off
Turn key "off" and wait 5 seconds
Repeat 2 more times for a total of 3 relearns.
Turn key off, then start it